Support default method bodies in #[devirt] trait definitions

[Kab1r]

Trait methods with default bodies (e.g. fn is_large(&self) -> bool { self.area() > 100.0 }) are now accepted by the proc-macro attribute.
The default body is placed on the inner trait's __spec_* provided
method, with self.method() calls rewritten to self.__spec_method()
via syn::visit_mut so sibling calls resolve within the inner trait.
Macro invocations (format!, write!, etc.) are handled via token-level
rewriting since syn doesn't parse macro token streams.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Organization UI

Review profile: ASSERTIVE

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Run ID: cd610a52-ab5b-4e76-a840-4358d0cf67ec

📥 Commits

Reviewing files that changed from the base of the PR and between e5f6142 and d73ec03.

📒 Files selected for processing (2)

• crates/core/tests/equivalence.rs
• crates/macros/src/lib.rs

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Traits can now include default method implementations that work seamlessly with devirtualization.
  • Default methods correctly dispatch through trait objects and common trait bounds (e.g., Send, Sync).
  • Implementations may override default methods and have those overrides respected.

• Tests

  • Added comprehensive tests and UI cases verifying default-method dispatch, overrides, and trait-object behavior.

Walkthrough

The PR enables trait methods with default bodies in the devirt macro by emitting inner-trait __spec_* declarations for required methods and rewriting self.method() call sites inside default bodies to invoke the inner-trait versions, plus applying the same rewrite to impl method blocks.

Changes

Cohort / File(s)	Summary
Dependency Updates Cargo.toml	Enabled syn feature visit-mut alongside full to allow AST mutation via syn::visit_mut::VisitMut.
Macro Core Implementation crates/macros/src/lib.rs	Removed validation that rejected default-bodied trait methods; generate inner __spec_* declarations for required methods and emit default method bodies rewritten to call self.__spec_*; added RewriteSelfCalls (uses syn::visit_mut::VisitMut) and token-level rewriting for macro token streams; apply same rewriting to impl method blocks.
Tests — integration/unit crates/core/tests/equivalence.rs	Added attr_defaults test module (behind #[cfg(feature = "macros")]) with structs/trait/tests exercising default-method dispatch, overrides, and trait-object combinations.
Tests — trybuild / UI crates/core/tests/ui_attr.rs, crates/core/tests/ui_attr/attr_default_*.rs	Registered three new passing UI tests and added three new test files validating default-body dispatch, default override behavior, and default methods with Send trait-object bounds.

Sequence Diagram

sequenceDiagram
    participant User as User Code
    participant Trait as Trait Def<br/>(with default methods)
    participant Macro as devirt Macro
    participant Inner as Inner Trait<br/>(__spec_*)
    participant Impl as Type Impl
    participant Runtime as Runtime Dispatch

    User->>Trait: Define trait with default method calling `self.get()`
    Trait->>Macro: #[devirt::devirt] annotation
    Macro->>Macro: Parse trait & default bodies
    Macro->>Macro: Rewrite `self.method()` → `self.__spec_method()` in default bodies
    Macro->>Inner: Emit `__spec_*` declarations for required signatures
    Macro->>Impl: Generate impls, rewriting impl blocks likewise
    User->>Runtime: Create &dyn Trait from instance
    Runtime->>Impl: Call default method
    Impl->>Impl: Execute rewritten body using `self.__spec_*()`
    Impl-->>Runtime: Return result
    Runtime-->>User: Result delivered

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• Emit dispatch shims for dyn Trait + Send/Sync/Send+Sync #17 — Modifies proc-macro expansion paths in crates/macros/src/lib.rs; touches similar codegen areas as this PR.
• Emit proc-macro expansion directly via quote! instead of delegating to __devirt_define! #16 — Adds __spec_* inner-trait generation and syn::visit_mut-based rewrites; strongly related to the default-body rewriting changes here.

Poem

🐰 I hopped through traits by moonlit code,
Rewrote each self-call down the road.
From self.call() to __spec_call() they glide,
Devirt packs defaults with pride. 🥕✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 40.63% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: adding support for default method bodies in the #[devirt] trait attribute macro.
Description check	✅ Passed	The description is directly related to the changeset, explaining the implementation details of how default method bodies are handled, including the rewriting mechanism for self method calls.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/default-method-bodies-YELZY

---

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@crates/core/tests/equivalence.rs`:
- Around line 149-206: The test attr_default_body_dispatch only exercises the
plain default-body path; add two additional runtime cases inside the same test:
(1) a case that exercises an overridden default implementation (create a new
struct e.g. DefOver with an impl Defaulted for DefOver that overrides is_big and
assert the overridden behavior via &dyn Defaulted), and (2) a case that
exercises a default that uses write!/format!-style code (create a struct e.g.
DefFmt that relies on the trait's default method which uses format!/write!
internally and assert its behavior via &dyn Defaulted and &(dyn Defaulted +
Send) to ensure macro-invocation rewriting is executed). Reference the existing
symbols Defaulted, DefHot/DefCold, and the test attr_default_body_dispatch when
adding these assertions.

In `@crates/macros/src/lib.rs`:
- Around line 320-328: expand_impl currently copies impl override method bodies
verbatim, so calls like self.area() inside an impl block don't get rewritten to
the generated sibling names; apply the same RewriteSelfCalls transformation used
in generate_spec_decls to those impl method blocks. Locate expand_impl and where
it captures the impl method body (the code that uses the impl method's
block/tokenstream), clone the Block into a mutable variable, instantiate
RewriteSelfCalls with the existing method_names collection, call
rewriter.visit_block_mut(&mut block) and then use the rewritten block when
quoting the generated __spec_* method instead of the original verbatim body so
sibling calls are renamed consistently.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: e82e59df-f8b5-488b-a38f-e81c825d2693

📥 Commits

Reviewing files that changed from the base of the PR and between d316853 and e5f6142.

📒 Files selected for processing (7)

• Cargo.toml
• crates/core/tests/equivalence.rs
• crates/core/tests/ui_attr.rs
• crates/core/tests/ui_attr/attr_default_body.rs
• crates/core/tests/ui_attr/attr_default_override.rs
• crates/core/tests/ui_attr/attr_default_send.rs
• crates/macros/src/lib.rs